Add search key and pseudo-port selector for port groups

[raimue (Rainer Müller)]

Similar to other field selectors it should be possible to select ports by which port groups they require. This should work with the port actions search and info as well as with a new pseudo-port selector.

Example Portfile:

PortGroup foo 1.0
PortGroup bar 1.5

name      blah
...

Expected commands and output:

$ port echo portgroup:foo
blah
$ port echo groups:bar
blah
$ port search --groups foo
blah @...
   ...
$ port info --groups blah
groups: foo 1.0, bar 1.5

The field PortInfo(portgroups) already exists. It is just not written into the PortIndex. It is currently a nested list of port groups and the corresponding version: {{foo 1.0} {bar 1.5}}.